VPLS N-PE redundancy with STP isolation

ABSTRACT

In one embodiment, a system includes a first network, a second network, and a core network connecting the first network to the second network. The first network includes a first set of two or more network devices, wherein the first network has a first spanning tree associated therewith. Similarly, the second network includes a second set of two or more network devices, wherein the second network has a second spanning tree associated therewith, wherein the second spanning tree is separate from the first spanning tree.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/355,668, filed Jan. 16, 2009, titled “VPLS N-PE REDUNDANCY WITH STPISOLATION,” the disclosure of which is hereby incorporated by referencein its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates generally to methods and apparatus forproviding redundancy within physically separate networks through the useof separate spanning trees.

2. Description of the Related Art

Today, companies often have offices that are geographically dispersed.Each of these geographical locations typically supports a separatephysical network. Each of these networks may provide access to data,applications, and other network resources. Such a network may bereferred to as a “data center.”

In order to support communication among multiple geographicallydispersed networks, companies often support a single Virtual Local AreaNetwork (VLAN). In order to support communication among thegeographically dispersed networks, a single spanning tree generated viathe Spanning Tree Protocol (STP) is often deployed for the VLAN over thegeographically dispersed networks.

With the increase of globally dispersed offices, the number of datacenters is increasing. Unfortunately, the STP may not be reliable over alarge number of hops and logical interfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example network in whichvarious embodiments of the invention may be implemented.

FIG. 2 is a diagram illustrating an example communication protocol thatmay be implemented by primary and backup network devices in accordancewith various embodiments.

FIG. 3 is a transaction flow diagram illustrating an examplecommunication protocol that may be implemented by primary and backupnetwork devices in accordance with various embodiments.

FIG. 4 is a diagrammatic representation of an example network device inwhich various embodiments may be implemented.

DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the disclosed embodiments.It will be obvious, however, to one skilled in the art, that thedisclosed embodiments may be practiced without some or all of thesespecific details. In other instances, well-known process steps have notbeen described in detail in order not to unnecessarily obscure thedisclosed embodiments.

Overview

In one embodiment, a system includes a first network, a second network,and a core network connecting the first network to the second network.The first network includes a first set of two or more network devices,wherein the first network has a first spanning tree associatedtherewith. Similarly, the second network includes a second set of two ormore network devices, wherein the second network has a second spanningtree associated therewith, wherein the second spanning tree is separatefrom the first spanning tree.

Specific Example Embodiments

Adding redundancy to an extended layer 2 network (e.g., Ethernetnetwork) is typically accomplished through the use of a spanning tree tokeep the topology free from loops. The Spanning Tree Protocol (STP) isan Open System Interconnection (OSI) layer-2 protocol that ensures aloop-free topology for any bridged LAN. STP allows a network design toinclude redundant links to provide automatic backup paths if an activelink fails, without the danger of bridge loops that may be created bybackup links, or the need for manual enabling/disabling of these backuplinks. It is important to prevent bridge loops, since they result inflooding the network.

Generally, the STP creates a spanning tree within a mesh network ofconnected Layer-2 bridges (e.g., Ethernet switches), and disables thoselinks that are not part of the tree, leaving a single active pathbetween any two network nodes.

There are a number of problems associated with the use of a singlespanning tree in association with an extended Layer 2 network that spansmultiple remote locations. Specifically, the STP does not providerobustness for large scale Layer 2 deployments. By means of exchangingbridge protocol data units (BPDUs) between network devices (e.g.,bridges), the STP elects the ports that eventually forward or blocktraffic.

Conservative default values for the STP timers typically impose amaximum network diameter of seven hops. Therefore, two bridges cannot bemore than seven hops away from each other. When a BPDU propagates fromthe root bridge of the spanning tree toward leave bridges of thespanning tree, an age provided in an age field of the BPDU incrementseach time the BPDU traverses a bridge. Eventually, the bridge discardsthe BPDU when the age exceeds the maximum age. Accordingly, convergenceof the spanning tree will be affected if the root bridge is too far awayfrom some bridges in the network.

An aggressive value for the maximum age parameter and the forward delaycan lead to a very unstable STP topology. In such cases, the loss ofsome BPDUs can cause a loop to appear.

Network stability may be compromised as a result of slow response tonetwork failures (slow convergence). Specifically, the STP is not builtto accommodate link flapping conditions, high error rates,uni-directional failures or non-report of loss of signal. These typicaland frequent problems associated with long and medium distance linkscould lead to STP slow convergence or instability.

One of the reasons for multiple network sites such as multi-site datacenters is disaster recovery. However, as data centers typically needLayer 2 connectivity, failure in one data center can affect other datacenters, which could potentially lead to a black-out of all data centersat the same time.

Due to the scaling problem of the STP, the disclosed embodiments supporta loop-free topology without the use of a spanning tree that spansmultiple physical networks (e.g., data centers). This may beaccomplished, in part, through the use of a separate spanning tree inassociation with each physical network (e.g., data center), rather thana single spanning tree that spans multiple physical networks. Aloop-free topology may be maintained through the use of a communicationprotocol that ensures that only a single active path is present betweenany two network nodes. In the following description, a systemarchitecture is set forth that enables multiple physical sites to beinterconnected while ensuring a loop-free topology.

FIG. 1 is a diagram illustrating an example system in which variousembodiments may be implemented. The system may include two or morephysical networks. In this example, the system includes three networks(e.g., data centers). Specifically, the system includes a first network102, Data Center A, a second network 104, Data Center B, and a thirdnetwork 106, Data Center C. The networks 102, 104, and 106 are coupledto one another via a core network 108 (e.g., provider network), whichmay maintain its own spanning tree. The networks 102, 104, and 106 maybe private networks, while the core network 108 may be a public network.The networks 102, 104, and 106 may be geographically separate such thatthey are coupled to one another through the core network 108.

The core network 108 may be a Virtual Private LAN Service (VPLS)network, which supports Ethernet based multipoint to multipointcommunication over Internet Protocol (IP) and Multi ProtocolMultiprotocol Label Switching (MPLS) networks. The VPLS allowsgeographically dispersed sites to share an Ethernet broadcast domain byconnecting the sites through pseudo-wires. In a VPLS, Local AreaNetworks (LANs) at each site (e.g., represented by the networks 102,104, and 106) may be extended to the edge of the provider network (e.g.,represented by the core network 108). The provider network may thenemulate a switch or bridge to connect all of the LANs to create a singlebridged LAN.

As shown in this example, each of the networks 102, 104, and 106 mayinclude a set of two or more network devices (e.g., bridges or routers).Specifically, each of the networks 102, 104, and 106 and correspondingset of network devices may include at least two redundant networkdevices (e.g., bridges or routers) connected to the core network 108.Since these redundant network devices are at the edge of the providernetwork, they may be referred to as Provider Edge (PE) devices (e.g.,routers). As shown in FIG. 1, the first network 102 includes redundantnetwork devices 110, 112, the second network 104 includes redundantnetwork devices 114, 116, and the third network 106 includes redundantnetwork devices 118, 120. The redundant network devices of each of thenetworks 102, 104, and 106 may each be connected to the core network108, as shown. In this example, the networks 102, 104, and 106 arephysical networks associated with separate physical sites. For instance,the networks 102, 104, and 106 may be storage area networks (SANs).

In accordance with one embodiment, each of the two redundant networkdevices of a particular physical network runs a communication protocol(e.g., redundancy protocol) such as a semaphore to prevent both of thetwo redundant network devices of the network from simultaneously beingin an active state, as will be described in further detail below.Specifically, the semaphores of a particular physical network may beannounced via at least one connection between the two redundant networkdevices of the physical network. In this example, the semaphores of thefirst network 102 may be announced via at least one connection betweenthe two redundant network devices 110, 112 of the first network 102 asshown at 122. Similarly, the semaphores of the second network 104 may beannounced via at least one connection between the two redundant networkdevices 114, 116 of the second network 104 as shown at 124, and thesemaphores of the third network 106 may be announced via at least oneconnection between the two redundant network devices 118, 120 of thethird network 106 as shown at 126.

At any given point in time, the redundant network devices of aparticular physical network include a single active network device andat least one backup network device. In accordance with variousembodiments, only one of the two redundant network devices forwardstraffic to and from the physical network at a given point in time. Theactive network device and the backup network device of each of thenetworks 102, 104, and 106 may each be connected to the core network108. In this example, the first network 102 has two redundant networkdevices 110, 112, the second network has two redundant network devices114, 116, and the third network has two redundant network devices 118,120. Specifically, the first network 102 includes an active networkdevice 110 and a backup network device 112, the second network 104includes an active network device 114 and a backup network device 116,and the third network 106 includes an active network device 118 and abackup network device 120.

It is important to note that should an active network device fail, thebackup network device would take over for the active device. In otherwords, the backup network device would process traffic on behalf of thefailed active device. The backup network device would then be consideredthe active device. Thus, a single network device may function as eithera backup network device or an active network device at any given pointin time.

An active network device may communicate with the backup network devicevia at least one connection between the active network device and thebackup network device. In addition, the active network device mayforward data packets to or from the core network 108 (e.g., between thecore network 108 and the physical network). In accordance with oneembodiment, traffic associated with the core network 108 cannot traversethe connection between the active network device and the backup networkdevice.

In contrast, a backup network device (i.e., standby network device) maycommunicate with the active network device via at least one connectionbetween the active network device and the backup network device.However, the backup network device may be incapable of forwarding datapackets to or from the core network 108 (e.g., between the core network108 and the physical network).

Each of the physical networks 102, 104, and 106 may have a separatespanning tree associated therewith. In accordance with one embodiment, aSTP domain is limited to a physical network (e.g., data center). Inother words, a spanning tree associated with a particular network is notassociated with other networks. Moreover, the spanning tree is notconnected to another spanning tree (e.g., associated with anothernetwork). The core network 108 need not implement a spanning tree.

The redundant network devices (e.g., bridges) of each of the physicalnetworks 102, 104, and 106 may run the Spanning Tree Protocol. To breakloops in a particular physical network (e.g., LAN) such as physicalnetworks 102, 104, and 106, the redundant network devices (e.g.,bridges) of that network may compute a spanning tree. The spanning treeallows a network to include redundant links to provide automatic backuppaths if an active link fails, without the danger of bridge loops or theneed for manual enabling/disabling of these backup links.

Another advantage of the use of separate spanning trees is the effect onTopology Change Notifications (TCNs). Typically, when a physicaltopology changes, STP convergence forces a TCN toward all domainswitches. Since each physical network has a separate spanning treedomain, TCNs are no longer required when a problem occurs in another,potentially distant, domain. Rather, TCNs may be transmitted only withinthe local STP domain.

Within each physical network, two redundant network devices each run acommunication protocol to prevent both of the redundant network devicesfrom simultaneously being in an active state. FIG. 2 is a diagramillustrating an example communication protocol that may be implementedby primary 202 and backup 204 network devices in accordance with variousembodiments. The communication protocol may be implemented by performinga two-way handshake. In accordance with one embodiment, thecommunication protocol may be implemented via semaphore signaling.Specifically, the primary network device 202 may implement a primarysemaphore 206 and the backup network device 204 may implement a backupsemaphore 208. Through implementing the primary semaphore 206 and thebackup semaphore 208, packets may be forwarded to and from the corenetwork via either a primary connection 210 between the primary networkdevice 202 and the core network or a backup connection 212 between thebackup network device 204 and the core network. The corresponding portsof the primary network device 202 and the backup network device 204 maybe set to forward or block traffic accordingly, as will be described infurther detail below with reference to FIG. 3.

FIG. 3 is a transaction flow diagram illustrating an examplecommunication protocol that may be implemented by primary and backupnetwork devices in accordance with various embodiments. Processesperformed by the primary and backup network devices will be describedwith reference to vertical lines 302 and 304, respectively. The primarynetwork device 302 may send a packet to the backup network device 304indicating that the primary network device 302 is up (e.g., active) asshown at 306. The primary network device 302 may wait a start-up delayat 308.

Upon receiving the packet indicating that the primary is up at 306, thebackup network device 304 may force the backup connection (e.g., port)to the core network down at 310 such that traffic to and from the corenetwork via the backup network device 304 is blocked. The backup networkdevice 304 may set the backup semaphore down at 312, and the backupnetwork device 304 may send a packet to the primary network device 302indicating that the backup network device 304 is down as shown at 314.Once the primary network device 302 receives the packet confirming thatthe backup network device 304 is not forwarding traffic, the primarynetwork device 302 sets the primary connection (e.g., port) up at 316such that traffic to and from the core network (e.g., between the corenetwork and the physical network) is forwarded via the primary networkdevice 302.

In the event that a failure occurs in the primary network device 302,this forces the primary connection down. More specifically, this isaccomplished by forcing the semaphore of the primary network device 302down. Through the use of a communication protocol such as the semaphoredescribed herein, only a single active network device will be incommunication with the core network.

Upon receiving a packet indicating that the primary is down at 318, thebackup network device 304 may set up the backup connection (e.g., port)to the core network at 320 such that traffic to and from the corenetwork via the backup network device 304 is not blocked. The backupnetwork device 304 may set up the backup semaphore at 322, and thebackup network device 304 may send a packet to the primary networkdevice 302 indicating that the backup network device 304 is active asshown at 324. Once the primary network device 302 receives the packetconfirming that the backup network device 304 is active (e.g.,forwarding traffic), the primary network device 302 forces the primaryconnection (e.g., port) down at 326 such that traffic to and from thecore network (e.g., between the core network and the physical network)is not forwarded via the primary network device 302.

Each of the redundant network devices associated with a particularphysical network may be configured with semaphores supporting operationas both a primary network device and a backup network device. Thisenables a backup network device to act as an active network device inthe event of failure of the backup network device. Moreover, once afailed active network device is brought up, it may act as a backupnetwork device.

Generally, the techniques for performing the disclosed embodiments maybe implemented on software and/or hardware. For example, they can beimplemented in an operating system kernel, in a separate user process,in a library package bound into network applications, on a speciallyconstructed machine, or on a network interface card. In a specificembodiment of this invention, the techniques of the present inventionare implemented in software such as an operating system or in anapplication running on an operating system.

A software or software/hardware hybrid packet processing system of thisinvention may be implemented on a general-purpose programmable machineselectively activated or reconfigured by a computer program stored inmemory. Such programmable machine may be a network device designed tohandle network traffic. Such network devices typically have multiplenetwork interfaces including frame relay and ISDN interfaces, forexample. Specific examples of such network devices include routers andswitches. A general architecture for some of these machines will appearfrom the description given below. Further, various embodiments may be atleast partially implemented on a card (e.g., an interface card) for anetwork device or a general-purpose computing device.

The disclosed embodiments may be implemented at network devices such asswitches or routers. Referring now to FIG. 4, a router or switch 1510suitable for implementing embodiments of the invention includes a mastercentral processing unit (CPU) 1562, one or more interfaces 1568, and abus 1515 (e.g., a PCI bus). When acting under the control of appropriatesoftware or firmware, the CPU 1562 is responsible for such router tasksas routing table computations and network management. It may also beresponsible for implementing the disclosed embodiments, in whole or inpart. The router may accomplish these functions under the control ofsoftware including an operating system (e.g., the Internetwork OperatingSystem (10S®) of Cisco Systems, Inc.) and any appropriate applicationssoftware. CPU 1562 may include one or more processors 1563 such as aprocessor from the Motorola family of microprocessors or the MIPS familyof microprocessors. In an alternative embodiment, the one or moreprocessors 1563 are specially designed hardware for controlling theoperations of router 1510. In a specific embodiment, a memory 1561 (suchas non-volatile RAM and/or ROM) also forms part of CPU 1562. However,there are many different ways in which memory could be coupled to thesystem. Memory block 1561 may be used for a variety of purposes such as,for example, caching and/or storing data, programming instructions, etc.

The one or more interfaces 1568 with one or more ports 1569 aretypically provided for one or more line cards 1570 (sometimes referredto as “interface cards”). In some embodiments, the one or more linecards1570 include memory such as registers 1572 and EEPROM 1574. Generally,the one or more linecards 1570 control the sending and receiving of datapackets or data segments over the network and sometimes support otherperipherals used with the router 1510. Among the interfaces that may beprovided are Ethernet interfaces, frame relay interfaces, cableinterfaces, DSL interfaces, token ring interfaces, and the like. Inaddition, various very high-speed interfaces may be provided such asfast Ethernet interfaces, Gigabit Ethernet interfaces, ATM interfaces,HSSI interfaces, POS interfaces, FDDI interfaces, LAN interfaces, WANinterfaces, metropolitan area network (MAN) interfaces and the like.Generally, these interfaces may include ports appropriate forcommunication with the appropriate media. In some cases, they may alsoinclude an independent processor and, in some instances, volatile RAM.The independent processors may control such communications intensivetasks as packet switching, media control and management. By providingseparate processors for the communications intensive tasks, theseinterfaces allow the master microprocessor 1562 to efficiently performrouting computations, network diagnostics, security functions, etc.Although the system shown in FIG. 4 is one specific router of thepresent invention, it is by no means the only router architecture onwhich the disclosed embodiments can be implemented. For example, anarchitecture having a single processor that handles communications aswell as routing computations, etc. is often used. Further, other typesof interfaces and media could also be used with the router.

Regardless of network device's configuration, it may employ one or morememories or memory modules (such as, for example, memory block 1565)configured to store data, program instructions for the general-purposenetwork operations and/or the inventive techniques described herein. Theprogram instructions may control the operation of an operating systemand/or one or more applications, for example.

Because such information and program instructions may be employed toimplement the systems/methods described herein, the disclosedembodiments relate to machine readable media that include programinstructions, state information, etc. for performing various operationsdescribed herein. Examples of machine-readable media include, but arenot limited to, magnetic media such as hard disks, floppy disks, andmagnetic tape; optical media such as CD-ROM disks and DVDs;magneto-optical media such as floptical disks; and hardware devices thatare specially configured to store and perform program instructions, suchas read-only memory devices (ROM) and random access memory (RAM).Examples of program instructions include both machine code, such asproduced by a compiler, and files containing higher level code that maybe executed by the computer using an interpreter.

Although illustrative embodiments and applications of the disclosedembodiments are shown and described herein, many variations andmodifications are possible which remain within the concept, scope, andspirit of the embodiments of the invention, and these variations wouldbecome clear to those of ordinary skill in the art after perusal of thisapplication. For example, the various examples described herein relateto the use of semaphores by the redundant network devices to ensure thatonly one of the redundant network devices is active at any given pointin time. However, the disclosed embodiments may also be performed usingother redundancy protocols or signaling mechanisms. Moreover, thedisclosed embodiments need not be performed using the steps describedabove. Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the disclosed embodiments are notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

What is claimed is:
 1. A system comprising: a first computer networkincluding a first plurality of network devices, wherein the firstcomputer network has a first spanning tree associated therewith; asecond computer network including a second plurality of network devices,wherein the second computer network has a second spanning treeassociated therewith, wherein the second spanning tree is separate fromthe first spanning tree; wherein a core network connects the firstcomputer network to the second computer network; wherein a first networkdevice of the first computer network is connected to a second networkdevice of the first computer network over an inter-device link, whereintraffic to and from the core network cannot be transmitted on theinter-device link; and wherein the first and second network devices areconfigured to: set, by the first network device, a primary semaphoreassociated with the first network device to an inactive state inresponse to a failure of the first network device; send, by the firstnetwork device, a first packet to the second network device via theinter-device link indicating that the the first network device is downin response to the failure of the first network device; set up, by thesecond network device, a second connection port connecting the secondnetwork device to the core network in response to receiving the firstpacket, thereby permitting traffic to and from the core network to beforwarded via the second network device; set, by the second networkdevice, a backup semaphore associated with the second network device toan active state in response to receiving the first packet; send, by thesecond network device, a second packet to the first network device viathe inter-device link indicating that the second network device is inthe active state; and force down, by the first network device, a firstconnection port connecting the first network device to the core networkin response to receiving the second packet, thereby blocking traffic toand from the core network via the first network device.
 2. The system ofclaim 1, wherein the core network is a public network.
 3. The system ofclaim 1, wherein the first network device runs the primary semaphore andthe second network device runs the backup semaphore to prevent both ofthe first and second network devices from simultaneously being in theactive state.
 4. The system of claim 1, wherein the primary semaphoreand the backup semaphore are announced via the inter-device link.
 5. Thesystem of claim 1, wherein each of the first and second network devicesruns a redundancy protocol to prevent both of the first and secondnetwork devices from simultaneously operating in the active state. 6.The system of claim 1, wherein, after resolving the failure of the firstnetwork device, the first network device is operated in a backup mode.7. A method comprising: setting, by a first network device of a firstcomputer network, a primary semaphore associated with the first networkdevice to an inactive state in response to a failure of the firstnetwork device, wherein the first computer network includes a pluralityof network devices including the first network device and a secondnetwork device, wherein the first and second network devices are eachcommunicatively coupled to a core network, and wherein the first andsecond network devices are communicatively coupled via an inter-devicelink over which traffic to and from the core network cannot betransmitted; sending, by the first network device, a first packet to thesecond network device via the inter-device link indicating that thefirst network device is down in response to the failure of the firstnetwork device; setting, by the second network device, a backupsemaphore associated with the second network device to an active statein response to receiving the first packet; setting up, by the secondnetwork device, a second connection port connecting the second networkdevice to the core network in response to receiving the first packet,thereby permitting traffic to and from the core network to be forwardedvia the second network device; sending, by the second network device, asecond packet to the first network device over the inter-device linkindicating that the second network device is in the active state; andforcing down, by the first network device, a first connection portconnecting the first network device to the core network in response toreceiving the second packet, thereby blocking traffic to and from thecore network via the first network device.
 8. The method of claim 7,wherein the core network is a public network.
 9. The method of claim 7,wherein the first computer network has a spanning tree associatedtherewith that is not associated with any other networks.
 10. The methodof claim 7, wherein the first computer network has a spanning treeassociated therewith that is separate from a spanning tree associatedwith a second computer network connected to the core network.
 11. Themethod of claim 7, wherein the second network device runs the backupsemaphore to prevent both of the first and second network devices fromsimultaneously being in the active state.
 12. The method of claim 7,wherein each of the first and second network devices runs a redundancyprotocol to prevent both of the first and second network devices fromsimultaneously operating in the active state.
 13. The method of claim 7,wherein, after resolving the failure of the first network device, thefirst network device is operated in a backup mode.
 14. A non-transitorycomputer readable storage medium including one or more programs, which,when executed by one or more processors of a first network device of afirst computer network and one or more processors of a second networkdevice of the first computer network, cause the first and second networkdevices to: set, by the first network device, a primary semaphoreassociated with the first network device to an inactive state inresponse to a failure of the first network device, wherein the first andsecond network devices are each communicatively coupled to a corenetwork, and wherein the first and second network devices arecommunicatively coupled via an inter-device link over which traffic toand from the core network cannot be transmitted; send, by the firstnetwork device, a first packet to the second network device via theinter-device link indicating that the first network device is down inresponse to the failure of the first network device; set up, by thesecond network device, a second connection port connecting the secondnetwork device to the core network in response to receiving the firstpacket, thereby permitting traffic to and from the core network to beforwarded via the second network device; set, by the second networkdevice, a backup semaphore associated with the second network device toan active state in response to receiving the first packet; send, by thesecond network device, a second packet to the first network device viathe inter-device link indicating that the second network device is inthe active state; and force down, by the first network device, a firstconnection port connecting the first network device to the core networkin response to receiving the second packet, thereby blocking traffic toand from the core network via the first network device.
 15. Thenon-transitory computer readable storage medium of claim 14, wherein thecore network is a public network.
 16. The non-transitory computerreadable storage medium of claim 14, wherein the first computer networkhas a spanning tree associated therewith that is not associated with anyother networks.
 17. The non-transitory computer readable storage mediumof claim 14, wherein each of the first and second network devices runs aredundancy protocol to prevent both of the first and second networkdevices from simultaneously operating in the active state.
 18. Thenon-transitory computer readable storage medium of claim 14, wherein thefirst network device runs the primary semaphore and the second networkdevice runs the backup semaphore to prevent both of the first and secondnetwork devices from simultaneously being in the active state.
 19. Thenon-transitory computer readable storage medium of claim 14, wherein,after resolving the failure of the first network device, the firstnetwork device is operated in a backup mode.
 20. The non-transitorycomputer readable storage medium of claim 14, wherein the primarysemaphore and the backup semaphore are announced via the inter-devicelink.